For example, compared to conventional materials for cutting tools such as carbide tools, etc., cBN based sintered body cutting tools have material characteristics of high performance that can be highly efficient and long-lasting because of the chemical stability and the extreme hardness of cBN sintered body. In addition, cBN sintered body cutting tools are highly valued for their superior flexibility and high environmental-friendly productivity compared to grinding tools as processing tools to deform such as cutting tools, and they have substituted for conventional tools in the machining of hard-to-cut ferrous materials.
cBN sintered body materials can be classified into two types: one type is a sintered body comprising cBN particles and binder materials, in which the cBN content ratio is high, the cBN particles bond each other, and the main components of the remainder are Co and Al as described in Patent Document 1, or is a sintered body that does not comprise any component other than cBN as much as possible, like Patent Document 2 (called “high cBN content ratio sintered body” hereinafter). The other type has a comparatively low cBN content ratio, has a low contact ratio between cBN elements, and is bonded through a ceramic comprising Ti nitrides (TiN) and carbides (TiC) that show a low affinity with iron, as disclosed in Patent Document 3 (called “low cBN content ratio sintered body” hereinafter).
In uses in which chips are split off and are not likely to generate shear heat, the former type, high cBN content ratio sintered body achieves outstanding stability and long lifetime because of the superior mechanical characteristics (extreme hardness, high strength, high toughness) and high thermal conductivity of the cBN; and it is suitable for cutting of ferrous sintered parts and gray cast iron in which mechanical wear and damage caused by rubbing against hardened particles and damage caused by thermal impact based on high speed interrupted cutting predominate.
Nonetheless, in machining of steel and hardened steel in which the cutting edge is exposed to a high temperature by large quantity of shear heat produced by continuous cutting, the lifespan is shorter than that of conventional carbide tools and ceramic tools because wear is rapidly advanced by the thermal wear of the cBN component with the iron.
Meanwhile, the latter, low cBN content ratio sintered body manifests superior wear resistance characteristics based on the workings of binder comprising TiN or TiC ceramic, which has a low affinity with iron at high temperatures, and in particular, in hardened steel machining which cannot be machined practically with conventional carbide tools and ceramic tools, low cBN content ratio sintered body has positively substituted in grinding as a cutting tool that can achieve a tool life ten to several dozen times that of conventional tools.
In recent years, by increasing rigidity of machine tools, adjusting the percentages of cBN and the Ti-based binder in low cBN content ratio sintered bodies, cBN sintered body tools are applied instead of grinding tools to machining applications in which the required precision is 3.2 μm to 6.3 μm in ten point averaged roughness (abbreviated “Rz” hereinafter), for example, as in cutting of automotive transmission parts and engine parts comprising hard-to-cut ferrous materials such as hardened steel, represented by SCM420, SCR420, S50C, and SUJ2, which are steels with a surface hardness heightened to Hv 4.5 GPa to 8.0 GPa (HRc45 to HRc64) by the so-called carburized hardening process, FCD (ductile) cast iron such as FCD600 with a hardness heightened to HB200 or more, and ADI (austempered) cast iron such as ADI1000.
Recently, in sliding surfaces and rotating surface, etc. that require a high precision surface roughness of Rz 0.4 μm to 3.2 μm, studies have begun on the application of cutting tools comprising low cBN content ratio sintered body instead of grinding, which has restrictions in terms of machining efficiency and flexibility, in uses for the final finishing step requiring high grade surfaces having sufficient fatigue strength in the machined region, or for semi-finish machining to obtain a high grade surface using only finish processing with an ultra-fine machining allowance of 5 to 10 μm or less, such as honing machine honing, which needs a smaller machining allowance than the conventional grinding process.
However, when cutting hardened steel with a machining efficiency of cutting speed V=100 m/min, depth of cut d=0.15 mm, and feed rate f=0.08 mm/rev. (chip removal volume W per unit time is 1,200 mm3/min) or more, which are judged to be beneficial to industrially apply cBN sintered body cutting tools to machining of hardened steel or high strength cast iron such as FCD and ADI, an affected layer by machining of a thickness of 1 to 20 μm may be formed on the surface of the machined part. The permissible range for the amount of this affected layer by machining produced is stipulated according to the required fatigue life characteristics, which depend on the various stress environments expected to be applied when the machined part is made to a final product.
Specifically, in cutting of universal joint or race bearing surfaces, which are roller and ball rotational track surfaces, if the thickness of aforementioned affected layer by machining is up to about several μm, this affected layer by machining may act as an extreme hardness protection film greater than the hardness by hardening process. If the thickness of affected layer by machining on the race surface of a bearing for uses applied high stress exceeds 10 μm, there is concern that the damage such as wear, flaking and peeling of the mating surface will be accelerated and the fatigue life will decrease, and therefore, in industry, machine processing is used in another process of time-consuming grinding to remove a machining allowance of several dozen μm.
It is known that when cutting after hardening the production of affected layer by machining increases the more that processing is conducted under high efficiency conditions. Nonetheless, the conditions producing the affected layer by machining and the characteristics of the affected layer by machining itself were not clear in detail.
Thus, for the hardened steel cutting evaluating a variety of cutting conditions using commercial cBN sintered body tools and then researching and analyzing the production of affected layers by machining revealed that the affected layer by machining in hardened steel cutting is composed of martensite as the main component, a mixed phase of residual austenite, bainite, iron oxide and an extremely small amount of iron nitride, etc. The affected layer by machining has a high hardness of about Hv9 GPa to 10 GPa, and is prone to have a tensile stress different from the residual stress of the hardened steel surface, on which the compression stress is supposed to remain principally, and ultimately in almost all cases the tensile stress remains on the machined surface if the affected layer by machining exceeds 10 μm.
The amount of the aforementioned affected layer by machining produced is serious when machining under high efficiency conditions or when an amount of cutting tool flank wear develops, and therefore, martensite produced on the machined part surface by hardening process changes phase to austenite by the heating during cutting caused by the continuous chip friction heat and the shearing heat, which is particular for hardened steel, as well as by heat during cutting due to the frictional heat between the machined surface of the machined part and the tool flank. And, a mixed phase having a main body of martensite including oxide phase and nitride phase is formed after cutting by rapid cooling in air including oxygen, nitrogen and water vapor. Consequently, when the cutting edge passes over the surface machined, the surface is exposed to high temperatures of at least 727° C. or more, which is the austenite transformation temperature of eutectoid steel, and therefore, a mechanism that selective plastic deformation arises on the outermost surface of the machined object by the thermal stress and the compression residual stress of the machined surface is offset works. The hypothesis here obtained is that if the machining surface is exposed to high temperatures at which the thickness of the affected layer by machining exceeds 5 μm, a tensile stress remains on the machined surface based on the mechanism and this tensile stress may lower the fatigue strength depending on the use of the machined parts.
Further, to clarify the characteristics required on the tool side to solve the problem, cutting was carried out by use of TiC—Al2O3 ceramic and cBN sintered body tools to cut SUJ2 test pieces hardened to a hardness of Hv 7 GPa, in order to evaluate differences in the thickness and form of affected layer by machining with the same cutting edge form, and the same cutting conditions at the time that the width of flank wear was the same. Irrespective of the time at which the width of flank wear was the same, an affected layer by machining was less likely to be produced in cBN sintered body tools than in ceramic tools, and it was revealed that, even if produced, the thickness of the affected layer by machining was ⅔ or less that of the ceramic tools. However, even when using cBN sintered body tools, if the thickness of affected layer by machining exceeds 10 μm, the residual stress was transformed from compression to tension in nearly all cases.
Derived from the aforementioned hypothesis regarding the mechanism of the production of residual stress, it is supposed that the cBN sintered body tool exhibiting a lower cutting edge temperature at the time of cutting influences. In order to further clarify this, the temperature of the cutting edge during cutting was measured using a two-color pyrometer, which can measure the temperature of micro-regions without being affected by the material of the tool or the condition of the tool surface, in the initial stage of cutting with no difference in the width of flank wear. The results unveiled that the cutting temperature in the cBN sintered body tool was 50% to 80% that in the ceramic tool, and the aforementioned hypothesis regarding the affected layer by machining generation mechanism in hardened steel cutting using a cBN sintered body tool was supported by the results obtained.
According to the results of the aforementioned research, in hardened steel cutting, it is necessary to lower the cutting edge temperature of the tool in order to improve the fatigue life of the machined parts, and as the simplest means of solution, it is effective to control the amount of heat generated during cutting by lowering the machining efficiency. However, when conducting a variety of studies with commercial cBN sintered body tools using ceramic binder materials comprising TiN or TiC for cutting hardened steel, irrespective of whether or not coolant is used, if machining efficiency is of a cutting speed V=70 m/min, depth of cut d=0.15 mm, and feed rate f=0.07 mm/rev. (chip removal volume W per unit time is 735 mm3/min) or more, even at the time of VB=0.1 mm, which is half of VB=0.2 mm, by the further temperature increase caused by the heat of friction when the cutting edge scrapes on the machined part, the value of the amount of flank wear VB generally determined to be the lifespan from the perspective of dimensional precision when cutting hardened steel, an affected layer by machining with a thickness of 10 μm is generated, the residual stress is tensile stress, and high efficiency machining, which is one of the great advantages of hardened steel cutting using cBN sintered body tools, becomes impossible.
Thus, it is necessary to develop a means to prevent residual tensile stress while keeping machining efficiency of a chip removal volume W per unit time of 1,200 mm3/min or more of one cBN sintered body insert, which is the general machining efficiency in hardened steel cutting using cBN sintered body tools.
Patent Document 1: Japanese Patent Publication No. S52-43486
Patent Document 2: Japanese Patent Publication No. H10-158065
Patent Document 3: Japanese Patent Publication No. S53-77811
Patent Document 4: Japanese Patent Publication No. H08-119774